1. Field of the Invention
This invention relates to GPS attitude determination and more specifically to a system and method of improved attitude determination using baseline extension.
2. Description of the Related Art
Attitude determination is of interest in such applications as aircraft and spacecraft control, artillery pointing, target designation and inertial navigation system attitude initialization. Attitude determination provides a “platform's” sight orientation within a reference coordinate system. Full attitude determination is typically specified in terms of yaw, pitch and roll. In some circumstances it is not necessary to disambiguate roll, this is referred to as pointing attitude. Typical solutions include flux compasses, inertial systems and GPS systems. Flux compasses are often used but are relatively inaccurate and easily disturbed by the environment, namely the presence of metal. Inertial solutions are expensive and have other problems.
GPS attitude determination systems can be either position based or interferometric. Position based systems use the geo-location readout (x,y,z) from two conventional GPS receivers connected to the pair of body-fixed antenna to calculate the attitude (yaw, pitch) of the baseline connecting the two GPS antennas. A third antenna or downfinder is required to determine the full-attitude including roll about the baseline. The orientation of the baseline (the line connecting the two GPS antenna) relative to the platform is fixed and known a priori. Thus, determination of the baseline attitude uniquely determines platform attitude. Interferometric systems use the geo-location read from one (or fuzed from both) of the conventional GPS receivers and calculate the attitude from the relative phases of the carriers for each satellite at both of the body-fixed antennas. Interferometric GPS is approximately an order of magnitude (10×) more accurate than position based GPS. A complete explanation of Interferometric GPS is provided in Paul Quinn “Instantaneous GPS Attitude Determination”, Proceedings of ION GPS-93, Part 1, Salt Lake City, Utah Sep. 22-24, 1993, pp. 603-615, which is hereby incorporated by reference.
In both the positional and interferometric GPS systems, the accuracy of the attitude determination is inversely proportion to the length of the baseline connecting the pair of body-fixed antenna (the error in locating the end points is roughly independent of the baseline length and the attitude error is the endpoint error divided by the length). Consequently, in fielded systems such as aircraft, satellites or targeting systems, the pair of antenna are typically mounted on the platform with the maximum practical separation at a known orientation to the platform, e.g. opposite corners of the platform. Quinn states “The attitude of a rigid body may be determined from the GPS phase observations from three or more antennas attached to the body” (P. 604, col. 1, lines 4-7). In describing a specific practical GPS attitude system, Quinn discloses that “All antennas were located on the corners of a one meter square” (P. 613, col 2, lines 13-15).